Fatigue behavior of spot friction welds in lap-shear and cross-tension specimens of dissimilar aluminum sheets

Abstract

Fatigue behavior of spot friction welds or friction stir spot welds in lap-shear and cross-tension specimens of dissimilar aluminum 5754-O and 7075-T6 sheets is investigated based on experimental observations and three fatigue life estimation models. Optical micrographs of the 5754/7075 and 7075/5754 welds before and after failure under quasi-static and cyclic loading conditions are examined to understand the fracture and failure mechanisms of the welds. Under cyclic loading conditions, the micrographs show that the 5754/7075 welds in lap-shear specimens mainly fail from the kinked fatigue crack through the lower sheet thickness. Also, the micrographs show that the 7075/5754 welds in lap-shear specimens mainly fail from the kinked fatigue crack through the lower sheet thickness and from the fracture surface through the upper sheet thickness. Under cyclic loading conditions, the micrographs show that the 5754/7075 and 7075/5754 welds in cross-tension specimens mainly fail from the fatigue crack along the interfacial surface and from the fracture surface through the upper sheet material. A kinked fatigue crack growth model based on the stress intensity factor solutions for finite kinked cracks, a structural stress model based on the closed-form structural stress solutions, and a through-nugget fatigue crack growth model based on the analytical stress intensity factor solutions for cracks growing through the weld nugget are adopted to estimate the fatigue lives of the 5754/7075 and 7075/5754 welds in lap-shear and cross-tension specimens. The fatigue life estimations based on the kinked fatigue crack growth model, the structural stress model and the through-nugget fatigue crack growth model agree well with the experimental results. Finally, the effective stress intensity factor and J integral solutions at the critical locations of the welds obtained from three-dimensional finite element analyses appear to be good fracture mechanics parameters to correlate the experimental fatigue data for the 5754/7075 and 7075/5754 welds in lap-shear and cross-tension specimens.